The present invention relates to a battery charging apparatus for a secondary cell.
Secondary cells mean batteries in which, even though they once charge current (so-called primary operation), they can be used again by returning to an original situation by means of charging (secondary operation). The secondary cells are also called storage batteries which are assembled to include two or more than two cells connected to one another in a manner to store electrical energy. Since it is possible to store energy again by charging the apparatus which once discharged the stored energy by discharging, it is possible to continuously use the assembly by repeating a charge/discharge cycle. There are well known batteries of this kind such as lead storage batteries, alkali storage batteries and cadmium storage batteries.
Two systems are conventionally known for charging systems for the secondary cell. One is a charging system for ordinarily causing current to flow in the secondary cell by operating the battery charging apparatus, and the other is a charging system installing a timer causing the charging system to start charging the secondary cell and to stop a charging operation after passing through a predetermined time by the timer.
In the former case, when the charging system is operated, the system continues supplying a charge current in one direction despite a condition of the secondary cell such as an over-discharge or under-discharge, thereby generating an overcharge so as to heat the cell. Accordingly, it is possible that heat causes a fire and gas occurring in the secondary cell can result in an explosion.
On the other hand, a charge generally starts under the condition that the discharge amount of the secondary cell to be charged is not detected. Therefore, in the latter system, when the discharge amount of the secondary cell is proper with respect to a set time (for example, thirteen hours) of the timer, there is no problem because an over-charge does not occur until a set time T.sub.1 as shown by graph h.sub.1 in FIG. 1. However, when the discharge amount is small, an overcharge condition can exist before the set time T.sub.1 passes through as shown by graph h.sub.2 in FIG. 1. For example, after ten hours from the start of charging, there is an overcharge condition (an oblique lined portion of the graph h.sub.2 in FIG. 1). Since the charging operation does not stop until the set time passes through, the secondary cell is heated and gas occurs, thereby resulting an danger of explosion of the secondary cell.